Method for induction sealing a plastic part to a composite container

ABSTRACT

A method for induction sealing a plastic part to a composite container using a pair of linear coils electrically connected to low frequency generators. The linear coils produce overlapping electromagnetic fields on either side of the container. Rotating the container assures even heating of the polyethylene liner and plastic part. The method produces a reliable seal between the liner and plastic part and can be used at high production speeds.

BACKGROUND

[0001] 1. Field of the Invention

[0002] This patent relates to a method for attaching a plastic part to acomposite container using induction heating. More particularly, thispatent relates to a method for induction sealing a plastic part to acomposite container by positioning the plastic part onto the top portionof the container and exposing the top portion of the container toelectromagnetic fields generated by a pair of linear shaped coils.

[0003] 2. Description of the Related Art

[0004] Composite containers are commonly used to hold drink mixes,snacks and other foods. A typical composite container has a cylindricalbody, a plastic or metal bottom and a plastic overcap. The containerbody is comprised of a polyfoil inner liner, a paperboard structurallayer and a paper outer label. The polyfoil inner layer has amoisture-proof thermoplastic layer that contacts the container contents,a metallic foil layer adjacent the thermoplastic layer, and an outerpaper layer. A plastic seal may be positioned under the plastic cap andsealed to the top edge of the container to help maintain the freshnessof the contents and extend its shelf life. Alternatively, the plasticcap may be sealed directly to the container body.

[0005] Sealing a plastic seal or cap to a composite container body isoften accomplished by heating the plastic seal or cap while pressing itonto the container body. Various means of heating the plastic seal orcap are known, including radiant heating and conductive heating, but themost successful means, at least with respect to composite containers, isinduction heating.

[0006] Induction heating is a process that relies on electrical currentswithin a material to produce heat. Induction heating requires a sourceof electrical energy, an induction coil and an electrically conductivetarget material (the material to be heated). As electricity passesthrough the induction coil, an electromagnetic field is generated aroundthe coil. The shape of the electromagnetic field depends on the shape ofthe coil. When the target material is exposed to the electromagneticfield, eddy currents are created in the target material, causing thetarget material to heat up because of resistance losses. The heat fromthe target material is transferred to adjacent plastic materials,causing them to soften and fuse together. The plastic materials form aseal upon cooling.

[0007] When sealing a plastic cap onto a composite container, it isdesirable to obtain a broad seal to prevent breaching of the seal. Abroad seal is defined as a seal at least ¼ inch wide, and preferably atleast ½ inch wide. It is also desirable to be able to seal containers athigh production speeds. High production speed is defined as at least 100containers per minute. Unfortunately, most commercially availableinduction sealing machines provide extremely localized heating. As aconsequence, it is difficult to obtain a broad seal between a plasticpart and a composite container at high production speeds.

[0008] Thus, it is an object of the present invention to provide amethod and apparatus for induction sealing a plastic part to a compositecontainer.

[0009] Another object is to provide a process for forming a broad sealbetween a plastic part and a composite container at faster speeds thanconventional induction sealing processes.

[0010] Further and additional objects will appear from the description,accompanying drawings, and appended claims.

SUMMARY OF THE INVENTION

[0011] The present invention is a method and apparatus for inductionsealing a plastic part to a composite container. The method includes thesteps of:

[0012] providing a composite container having a body, the body having atop portion terminating in a top edge, the body defining a cylinderextending beyond the top edge, the body comprising an outer structurallayer, an intermediate layer of electrically conductive material and aninnermost layer of thermoplastic adjacent the electrically conductivelayer;

[0013] positioning a plastic part onto the composite container such thatit forms a friction fit with the inside of the top portion of thecontainer body;

[0014] exposing the top portion of the container body to a pair ofelectromagnetic fields created by a pair of induction coils arranged inparallel on either side of the container, each coil doubling back onitself so as to form two substantially parallel linear sections, onelinear section of each coil being positioned above the container topedge and intersecting the cylinder defined by the container body, theother linear section being positioned below the top edge and lateral tothe container body, whereby said exposure induces an electrical currentin the electrically conductive layer of the container body, therebygenerating heat that raises the temperature of the adjacentthermoplastic layer to a temperature suitable for adhering thethermoplastic layer to the plastic part;

[0015] rotating the container while it is being exposed to theelectromagnetic fields; and

[0016] forming a seal between the plastic part and the thermoplasticinner layer of the container by removing the top portion of thecontainer from the electromagnetic fields and allowing the thermoplasticlayer to cool and solidify.

[0017] The parallel linear sections of each coil create overlappingelectromagnetic fields. As the container is exposed to the overlappingelectromagnetic fields, the container is carried on a moving conveyorbelt and rotated to assure even heating of the thermoplastic liner andplastic part.

THE DRAWINGS

[0018]FIG. 1 is a perspective view of a detail of an induction sealingapparatus used in the present invention, showing three compositecontainers as they are conveyed through the apparatus.

[0019]FIG. 2 is a cross sectional view taken along line 2-2 of one ofthe containers of FIG. 1.

[0020]FIG. 3 is an end plan view of the induction sealing apparatus ofFIG. 1.

[0021]FIG. 4 is a side plan view of the induction sealing apparatus ofFIG. 1.

[0022]FIG. 5 is a top plan view of the induction sealing apparatus ofFIG. 1.

DETAILED DESCRIPTION OF THE INVENTION

[0023] The present invention is a method for induction sealing a plasticpart, such as a lid, to a composite container. In brief summary, themethod involves the following steps. First, a composite container isprovided having a foil layer and an adjacent thermoplastic inner layer.The plastic part is positioned onto the top portion of the container sothat it forms a friction fit with the thermoplastic inner layer. Thecontainer and plastic part are then passed through an induction sealingapparatus via a conveyer belt or other suitable means. As the containerpasses through the induction sealing apparatus, the top portion (¼″-½″)of the container is exposed to two pairs of overlapping electromagneticfields created by a pair of linear induction coils electricallyconnected to alternating current generators and positioned on oppositesides of the container. The electromagnetic fields induce eddy currentsin the foil layer, causing the foil to heat up. The foil heats theadjacent thermoplastic layer and may also heat the plastic part, causingthe thermoplastic layer and plastic part to soften and fuse together.Finally, the thermoplastic layer and plastic part are cooled, forming aseal between the two.

[0024] Turning to the figures, there is shown in FIG. 1 a perspectiveview of a detail of an induction sealing apparatus 10 used in thepresent invention. The figure shows three composite containers 12 beingconveyed through the induction sealing apparatus 10. As perhaps bestshown in FIG. 2, each container 12 comprises a cylindrical body portion14 and a bottom 16. The bottom 16 may be made from plastic, metal or anyother suitable material. The body 14 has an open top end (before a cap24 is positioned onto the container) and terminates in a top edge 18.The body wall 14 comprises a polyfoil inner liner 20 and a paperboardstructural outer layer 22. An optional printed-paper label (not shown)may be wrapped around the paperboard layer 22 and adhered thereto.

[0025] The polyfoil inner liner 20 typically comprises, from the insideout, a thermoplastic layer (typically polyethylene or polypropylene), anelectrically conductive layer (typically a metallic foil such asaluminum foil), a second thermoplastic layer and a paper layer, thepaper layer adjoining and bonded to the paperboard structural layer 22of the container body 14. For the purposes of the description and claimsto follow, the cylinder defined by the container body 14 may be thoughtof as extending beyond the top edge 18 of the body 14.

[0026] A plastic part 24 is positioned onto the composite container 12such that the plastic part 24 forms a friction fit with the inside ofthe top portion 26 of the container body 14. The plastic part 24 may bea lid, as in the illustrated example, and may have incorporated into itsdesign such features as a spout, sifting holes or other dispensingfeature. As best shown in FIG. 2, the plastic part 24 has a coveringportion 25 and a sidewall 27 extending downward from the coveringportion 25. The outer diameter of the sidewall 27 is only slightlysmaller than the inner diameter of the container body 14 to provide goodradial contact between the plastic part 24 and the cylindrical body 14.The plastic part 24 has an annular flange 28 extending radially from thesidewall 27 that abuts the top edge 18 of the container 10 when theplastic part 24 is fully seated on the container 10.

[0027] Critical to the success of the method is the unique form andplacement of the induction coils 30, 32. Known methods for inductionsealing parts, such as that disclosed in Japanese Patent ApplicationNumber 02125767, use serpentine coils placed above the mouth of thecontainer. This serpentine configuration is commonly used for inductionsealing prescription medicine bottles and caps. However, serpentinecoils can result in hot spots in the metal foil, which can causescorching of the paper layers and a poor seal.

[0028] The present method uses a linear coil configuration. Although themethod would work with a single linear coil, a pair of coils 30, 32arranged in parallel on either side of the container 12 is preferred forfaster sealing and thus increased productivity. Each coil doubles backon itself to form two substantially parallel linear sections. Forexample, the right side coil 30 in FIG. 1 has a first linear section 36and a second linear section 38 substantially parallel to the first.Preferably, the linear sections 36, 38 are about sixteen inches long,and are electrically connected to an alternating current generator. Eachcoil may be electrically connected to its own generator.

[0029] The linear sections of each coil 30, 32 create overlappingelectromagnetic fields that allow for more even heating of the polyfoilinner liner 20, and thus, less chance of scorching. To achieve a ¼″-½″wide seal, each coil 30, 32 preferably is arranged such that one lengthof the coil is located about {fraction (1/10)} inches above the planedefined by the top edge 18 of the container 12 and within thecylindrical area defined by the container body 14, as best shown inFIGS. 2 and 5. The other length of the coil is located about {fraction(1/10)} to ¼ inches lower than this plane, as perhaps best shown inFIGS. 1 and 2.

[0030]FIGS. 3 and 4 show two views of an apparatus 10 for inductionsealing a plastic part to of a container 12 according to the presentinvention. The containers 12 move along a conveyor belt 42 and areturned at least two times, and preferably about 2.1 times, by a screwdrive 44 or other rotating means as they pass by the sixteen-inch longcoils 30, 32. Because of the dual coil configuration, with every180-degree turn of each container 12, the entire circumference of thecontainer 12 near the top edge 18 is exposed to the electromagneticenergy fields generated by the coils 30, 32.

[0031] The coils may be raised or lowered to accommodate containers ofvarying heights. A relatively shorter container 12′ is shown in phantomin FIG. 3 to demonstrate this capability.

EXAMPLE

[0032] Molded polyethylene plastic lids were sealed onto compositecontainers using an induction sealing apparatus. The containers had abody comprising a polyfoil inner liner, a structural layer made fromrecycled paper, and an outer label made from paper and bearing graphics.The polyfoil inner liner included a layer of aluminum foil 0.00035inches thick. The plastic lids were positioned onto the compositecontainer so that they formed a friction fit with the inside of the topportion of the container body.

[0033] Normally, thin foils such as 0.00035 inch-thick aluminum foiltypically used in composite containers do not heat well with lowfrequency generators (generators that generate frequencies lower than450 KHz). Surprisingly, we found that excellent bonding between thethermoplastic liner and the plastic part could be achieved usinggenerators that generate frequencies in the range of 80-200 KHz, such asthe 15 KW generators manufactured by Lepel Corporation.

[0034] The containers were placed on a conveyer belt and passed throughthe induction sealing apparatus. The apparatus comprised a pair ofinduction coils arranged in parallel. Each of the induction coilsdoubled back on itself and formed two substantially parallel linearsections about 16 inches long. One linear section of each coil waspositioned so that, as the container passed through the apparatus, thesections were about 0.1 inches above the container top and directly overthe container. The other linear sections were positioned about ½ inchbelow the top of the containers and lateral to the containers.

[0035] As the containers passed through the induction sealing apparatusthe top portions of the containers were exposed to a pair of overlappingelectromagnetic fields on either side of the container top. To assurecomplete exposure of the container tops to the electromagnetic fields,and to allow the polyethylene liner to attain the correct temperatureand to fuse with the plastic part, the containers were rotated about 2.1times each as they passed through the electromagnetic fields. No extrapressure was placed on the plastic parts before, during or after themovement of the container through the induction sealing apparatus. Uponexiting the apparatus, the containers were allowed to cool and the sealswere tested for leakage. The resulting containers were found to haveexcellent seals. About 100 containers per minute could be sealed in thisway.

[0036] Thus there has been described a method for induction sealing aplastic part to a composite container using a pair of linear coilselectrically connected to low frequency generators. The linear coilsproduce overlapping electromagnetic fields on either side of thecontainer. Rotating the container as it is exposed to theelectromagnetic fields assures even heating of the thermoplastic linerand plastic part. The method produces a reliable seal between the linerand plastic part and can be used at high production speeds.

[0037] Other modifications and alternative embodiments of the inventionare contemplated which do not depart from the spirit and scope of theinvention as defined by the foregoing teachings and appended claims. Itis intended that the claims cover all such modifications that fallwithin their scope.

We claim as our invention:
 1. A method for induction sealing a plasticpart to a composite container, the method comprising the steps of:providing a composite container having a body, the body having a topportion terminating in a top edge, the body defining a cylinderextending beyond the top edge, the body comprising an outer structurallayer, an intermediate layer of electrically conductive material and aninnermost layer of thermoplastic adjacent the electrically conductivelayer; positioning a plastic part onto the composite container such thatit forms a friction fit with the inside of the top portion of thecontainer body; exposing the top portion of the container body to a pairof electromagnetic fields created by a pair of induction coils arrangedin parallel on opposite sides of the container, each coil doubling backon itself so as to form two substantially parallel linear sections, onelinear section of each coil being positioned above the container topedge and intersecting the cylinder defined by the container body, theother linear section being positioned below the top edge and lateral tothe container body, whereby said exposure induces an electrical currentin the electrically conductive layer of the container body, therebygenerating heat that raises the temperature of the adjacentthermoplastic layer to a temperature suitable for adhering thethermoplastic layer to the plastic part; rotating the container while itis being exposed to the electromagnetic fields; and forming a sealbetween the plastic part and the thermoplastic inner layer of thecontainer by removing the top portion of the container from theelectromagnetic fields and allowing the thermoplastic layer to cool andsolidify.
 2. The method of claim 1 wherein each coil is electricallyconnected to a separate electrical generator.
 3. The method of claim 1wherein one linear section of each coil is about 0.1 inches above thetop edge of the container body and the other linear section is about 0.1to about 0.5 inches below the top edge.
 4. The method of claim 3 whereinthe other linear section is about 0.1 to 0.25 inches below the top edge.5. The method of claim 1 wherein the linear sections are about sixteeninches in length.
 6. The method of claim 1 wherein, during the exposingstep, the container is carried on a moving conveyor belt past the coils.7. The method of claim 1 wherein the container is rotated by a screwdrive.
 8. The method of claim 1 wherein the structural layer ispaperboard.
 9. The method of claim 8 wherein the electrically conductivelayer is metallic foil.
 10. The method of claim 9 wherein the metallicfoil is aluminum foil.
 11. The method of claim 1 wherein the plasticpart is a lid.